The present invention relates to an image forming apparatus for inputting an image to convert the input image into image data necessary for image formation, and more specifically, it relates to an image forming apparatus for performing a thinning process.
In a copying machine, printing rate is one of the factors that determine toner consumption per transfer paper. The printing rate is a value that is determined by the total area of printed objects in a copy image of a specific evaluation chart, and it is intimately concerned with the number of dots in an image formed by an image forming unit. Therefore, if the line of each letter or the like can be thinned by image processing, the number of dots in the image can be decreased. It is thus considered that thinning the lines by image processing to decrease the printing rate is effective for reduction of toner consumption.
Heretofore, thinning (line width control) has been performed after pseudo gradation processing such as error diffusion. As a discrimination method between an edge portion and a non-edge portion in an image, a matching process with a template pattern prepared in advance has been performed.
In the above-mentioned prior art, however, since the target is binary image data after the gradation processing, discrimination accuracy is limited, and erroneous discrimination also occurs in places. Therefore, if the degree of thinning is increased, a non-edge portion in a gradated photograph or the like may erroneously be discriminated as an edge portion and in such a case, an undesired influence occurs wherein the pixels in that portion are thinned. There is a problem that the degree of thinning can not sufficiently be increased to avoid such an influence. Besides, such an edge discrimination process by pattern matching requires a large amount of calculation and it is complicated. This causes another problem of an increase in circuit scale.